they used an al qaeda internet description of how to make a pressure cooker bomb. seth jones, a counterterror expert at the rand corporation, has analyzed the marathon attack and how it was pulled off. >> ad libbed part of it and made some decisions on a few elements of the bomb making. but what's different about this is they took a simple recipe and then targeted a -- the boston marathon. >> reporter: why the marathon? because, officials say, it was there, essentially. and easy. not long in the planning either. >> it does appear though that the younger brother had not become involved until later in the process, perhaps a week or so before the actual attack. >> reporter: jones says the brothers seemed to have been inspired by the internet preachings of anwar al awlaki, the charismatic american-born al qaeda jihadist who has been dead now more than a year and a half. >> the older brother appeared to be the more radicalized of the two and drove both the need to conduct the attack as well as the preparation of the attack, that is, building the bombs. >> reporter: as f